1. Field of the Invention
This invention relates to transformer coil bodies and more particularly to a molded transformer coil body or bobbin.
2. Description of the Prior Art
In the United States transformer coil bodies are normally formed of plastics materials. Two primary types are used, those having axially separated winding areas and those having radially separated winding areas. Such coil bodies may have primary and secondary winding areas, two primary and a single secondary, or two secondary and a single primary. In standard U.S. bodies the primary area and secondary areas are normally separated by a single wall member. A common design is to provide a core tube, normally rectangular in cross-section and a plurality of outwardly projecting flange walls axially spaced along the length of the core tube to define winding areas between the walls radially of the core tube. Such coil bodies are formed, as desired, with other projecting members such as mounting members, pin members, etc. Commonly, the entire coil body is formed of a single molded plastic piece. Existing winding machines are designed to accommodate such single piece coil bodies.
On the other hand, standards of other countries, such as the countries of the European common market, have adopted standards providing for a minimum creepage distance between the primary and secondary windings. Although such minimum creepage distances can be established by making the projecting walls between the winding areas of a height from the outermost coil winding equal to at least half the minimum creepage distance whereby the entire minimum creepage distance will be established as the creepage path up one side of the wall from the winding of the one coil thence down the other side of the wall to the winding of the other coil, such a solution is undesired. An extended wall solution would require a greater than desired height for the overall transformer which height would be made up mostly of wasted space, i.e. the space outward from the outermost winding. Since the metal laminates of the transformer are commonly E-shaped having a central tang in the core tube and legs extending axially of the coil body immediately outwardly of the walls, such a solution would provide a greater spacing between the legs and tang than is otherwise desired.
It has been known to solve the problem of the minimum creepage path by providing a separate T-shaped cross-section member between individual bodied portions with one body portion used for the primary winding coil and a second body portion used for the secondary winding coil. The leg of the T is positioned between the two body portions with the cross bar of the T overlying each of the body portions. Both the leg and the cross bar are then dimensioned to be of a length such that the creepage path around the leg or around the cross bar from the outermost coil winding is equal to the minimum creepage distance established by various standards.
While such solutions have overcome the height problem, they complicate manufacture and assembly of the transformer since individual winding areas must be provided as separate parts. This is required because once the cross bar extends over the winding area, the coil could not be wound. Therefore, the coils must be wound around the individual body parts prior to assembly with the T-shaped cross-section member.
It would therefore be an advance in the art to provide a transformer coil having an elongated creepage path between primary and secondary windings without increasing the height of the coil body around the entire periphery thereof and without requiring separate coil bodies for the primary and secondary windings.